Final answer:
A line graph is the most effective format for illustrating a relationship between two variables that are both changing, such as time series graphs showing patterns over time.
Step-by-step explanation:
A line graph is often the most effective format for illustrating a relationship between two variables that are both changing. For example, time series graphs can show patterns as time changes, like the unemployment rate over time.
The graphs we have discussed so far are called line graphs, because they show a relationship between two variables: one measured on the horizontal axis and the other measured on the vertical axis.